Culture techniques for bone marrow culture of normal hematopoietic cells have been established in the last two years in our laboratory. Using these techniques, non-tumorigenic bone marrow cells from W/Wv and S1/S1(d) mice, which have a deficiency in the CFU-s or CFU-c compartment, have been maintained for more than 6 months. These mutant mice lack mast cells in skin and connective tissues. However, the established cell lines in vitro showed metachromasia by toluidine blue stain and had histamine and receptors for IgE, strongly indicating that these mice have at least committed stem cells for basophil/mast cells. The cell lines also showed CFU-META in semi-solid culture. After infection by oncogenic type C viruses, virus producing basophil/mast cell lines from NFS/N mice have been maintained for more than 3 months. Examination of in vitro transformation in suspension culture systems is now in progress.